Smooth bromegrass (Bromus inermis Leyss.) is an introduced, perennial, cool-season invasive grass that has invaded native rangelands in the Great Plains. Defoliation at specific growth stages may reduce the abundance of B. inermis but information is limited about when this should occur. Between 2018 and 2020 we assessed how defoliation at four different phenological stages influenced the amount of outgrowth on B. inermis tillers near Mandan, North Dakota, USA. In three replicated plots we evaluated axillary buds, tillers, and rhizome outgrowth. Each year, 10 tillers per plot were defoliated at a height of 5 cm at each of the following phenological stages: 1) once in the vegetative stage; 2) twice in the vegetative stage, 3) once in the elongation stage, 4) once in the reproductive stage, and an undefoliated control. Individual tillers were collected in the fall following defoliation and processed in the laboratory. The total number of crown positions were determined for each tiller and outgrowth (tillers and rhizomes) and number of axillary buds were counted on each tiller. A double staining technique was used to determine active, dead and dormant axillary buds. Tillers defoliated twice in the vegetative stage had less outgrowth per tiller than tillers defoliated once in the reproductive stage or the undefoliated controls (new outgrowth of 1.2, 2.2, and 1.8 per tiller for twice vegetatively, reproductive and control respectively). Results show that defoliating tillers in the elongation or reproductive stage resulted in nearly complete tiller mortality. Our data suggest managers who wish to reduce B. inermis abundance should focus on defoliating it twice in the vegetative stage and avoid defoliating B. inermis in the reproductive stage to limit outgrowth.